The present invention relates to photosensitive elastomeric compositions used to prepare digitally imaged relief-printing plates without the need for an interim process step.
Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only be means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
A typical flexographic printing plate as delivered by its manufacturer, is a multilayered article made of, in order, a backing or support layer, one or more unexposed photocurable layers, a protective layer or slip film, and a cover sheet. The backing layer lends support to the plate. It is typically a plastic film or sheet about 5 mils or so thick, which may be transparent or opaque. Polyester films, such as polyethylene terephthalate film, are examples of materials that can be suitably used as the backing. When only a single photocurable layer is present, it may be anywhere from about 25-275 mils thick, and can be formulated from any of a wide variety of known photopolymers, initiators, reactive diluents, etc. In some plates, there is a second photocurable layer (referred to as an xe2x80x9covercoatxe2x80x9d or xe2x80x9cprintingxe2x80x9d layer) atop this first, base layer of photocurable material. This second layer usually has a similar composition to the first layer, but is generally much thinner, being on the order of less than about 10 mils thick. The slip film is a thin (0.1 to 1.0 mils) sheet, which is transparent to UV light, which protects the photopolymer from dust and increases its ease of handling. The cover sheet is a heavy, protective layer, typically polyester, plastic, or paper. Typical prior art methods for making flexographic printing plates may be found, for example, in U.S. Pat. Nos. 4,045,231, 5,223,375 and 5,925,500, the teachings of which are incorporated by reference herein in their entirety.
It is highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of plates in developing relief images, in order to go from plate to press more quickly. An early attempt to reduce solvents, and the inherently longer drying required for solvent developing was the aqueous developable flexographic printing plate as taught in U.S. Pat. Nos. 4,177,074, 4,517,279, 5,364,741 and 6,017,679, the teachings of which are herein incorporated by reference in their entirety. However, the use of water to develop relief is still a xe2x80x9cprocessingxe2x80x9d step. In addition, water developable printing plates have inherent disadvantages, such as limited print performance and the generation of wastewater.
Thermal mass transfer plates, such as DuPont Cyrel(copyright) FAST(trademark), are gaining popularity because they are chemical free. In the case of the FAST(trademark) approach, the thermal process of removing the uncured non-image areas of the photopolymer is carried out after cross-linking the image areas of the plate. This approach is demonstrated in U.S. Pat. No. 6,171,758, and in Patent Nos. WO0118604 and WO0188615, the teachings of which are herein incorporated by reference in their entirety. Since the photopolymer is xe2x80x9cdensexe2x80x9d, removing of the uncured non-image areas takes a substantial amount of time to achieve. Customers must also invest in a special proprietary processor.
Laser-engraving systems from Fulflex and BASF (called LEP) are also process-free. An example of this technology is found in Patent No. EP0982124A2 the teachings of which are herein incorporated by reference in their entirety. In the BASF and ZED/Fulflex approach, the photopolymer/rubber is cured or cross-linked prior to the engraving step. Once again, because of the high density of these materials, the thermal engraving step is long and tedious. Additionally, high resolution is difficult to achieve. Thus, the disadvantage of prior art engraved plates is a combination of limited resolution and throughput.
Directly engraving a relief plate with a laser is a highly desirable concept. However, CO2 engraving lasers lack beam resolution and cause anomalies due to heat dissipation. The resolution of such systems is limited to well below 133 lines per inch (LPI) on a practical basis. Infrared (IR) lasers such as Nd-YAG lasers are extremely high in resolution and are precisely controlled. However, these lasers lack the necessary power and reactivity to engrave conventional photopolymers and may be too slow due to mass transfer limitations in dense xe2x80x9ccuredxe2x80x9d photopolymer or rubber systems.
A solution to the problem may lie in the use of a UV-curable thermoplastic elastomer that contains micro-bubbles. The composition is essentially a photocurable elastomeric uncured foam that is laden with a dye that is both IR absorbing and UV transmissive. As the IR laser strikes the dye, it transfers IR energy into heat, causing xe2x80x9claser collapsexe2x80x9d of the micro-bubbles or microspheres. Because the photocurable elastomeric material consists of foam cells which are only microns in size, the ablation-to-depth process can occur much more quickly, using much lower energy than is required in true mass transfer systems such as mask ablation or polymer engraving. In addition, the lower density and the corresponding lower heat energies involved in this process act to prevent conductance of heat energy to adjoining cells, thus limiting thermal damage and having the potential for higher resolution than traditional laser engraving. After all of the non-printing (relief) areas have been laser collapsed, there may be an additional process step to laser collapse the top layer to form a denser printing surface. This denser printing surface can also be created by a xe2x80x9cbumpxe2x80x9d UV-exposure in concert with the regular exposure. A xe2x80x9cbumpxe2x80x9d or xe2x80x9cflashxe2x80x9d exposure refers to a quick exposure, generally of less than about 1 second. The photopolymer is then flood UV-exposed to cross-link the formed image for enhanced physical properties. Finally, the process may contain a detacking step.
The advantage of this xe2x80x9clow densityxe2x80x9d approach is that it may be used in any of the conventional plate-setters in the industry, with only a change in the software that is used to control the energy density; no major investment in hardware is needed. The disadvantage of UV-imaging through a xe2x80x9cfoamxe2x80x9d is obviated because the imaging is done by the interaction of the IR laser with the microspheres. UV-curing is used simply to set the image in place. Furthermore, in using this process, one avoids the washout process step, and hence has the workflow advantage of going from the plate to press much more quickly than in conventional flexographic printing plates, while at the same time reducing solid waste generation.
U.S. Pat. No. 6,159,659 and U.S. Pat. No. 6,090,529, both to Gelbart, the teachings of which are incorporated herein by reference in their entirety, disclose methods for directly creating a raised image on a flexographic printing surface. These patents disclose laser ablation of an intermediate layer that comprises an elastomer and a high concentration of plastic or glass microballoons, in order to form recessed areas on the surface. In addition, these patents disclose controlling the intensity of the laser beam and the dwell time of the laser beam in each spot so that the laser power applied to each part of the surface is sufficient to cause localized melting of the intermediate layer. The dwell time is sufficiently long so as to produce viscous flow of the melted material, while the laser intensity is insufficient to cause complete ablation of the intermediate layer. In one example, the printing plate is made from a closed-cell black polyurethane foam, where the foam has a density of about 10% that of solid polyurethane. U.S. Pat. No. 6,159,659 further discloses that when the plate is xe2x80x9ccutxe2x80x9d or ablated, with a laser at the operating wavelength, the cutting action is self-limiting because of the insensitivity of the backing at the operating wavelength, which avoids damage to the backing.
The present invention comprises a collapsible photosensitive elastomer composition comprising a UV-curable elastomer, an infrared dye, and microspheres. In contrast, the U.S. Pat. No. 6,090,529 and the U.S. Pat. No. 6,159,659 do not disclose a photosensitive elastomer and do not disclose crosslinking the composition of the formed image in order to enhance the physical properties of the printing plate. In addition, the U.S. Pat. No. 6,090,529 and the U.S. Pat. No. 6,159,659 do not disclose how the foam intermediate layer behaves as a printing plate. The patents disclose that the pigment/dye is carbon-based, which is not suitable for use in the present invention because it will interfere with the photocurable aspects of the invention.
The present invention relates to a UV cross-linkable raw material that is cured after laser imaging for added physical strength necessary for press life durability. The present invention also advocates the use of microspheres, which will give excellent image fidelity and consistency. As explained in more detail below, the choice of the microspheres and the laser dye is key to the success of this invention.
The new concept of the present invention addresses the market need for eliminating the need for chemical processing of printing plates, by using a very low-density photopolymer plate that is impregnated with infrared (IR) sensitive micro-bubbles, that collapse when irradiated with an IR laser. Subsequently the photopolymer can be UV-cured to cross-link the material for enhanced physical properties.
It is an object of the present invention to provide a printing plate comprising a collapsible UV-curable elastomer composition comprising a UV-curable elastomer, an infrared dye, and microspheres and a method for making a printing plate, which does not require the use of any interim process steps.
Another aspect of the invention is to provide a method of making a laser imageable printing plate using a collapsible UV cross-linkable material that comprises microspheres, so as to decrease the final density of the plate formulation.
The objects of the invention can be accomplished by providing a digitally imaged relief printing plate and a method of making the digitally imaged relief-printing plate comprising the steps of
a) extruding a collapsible UV-curable elastomer composition comprising (i) a UV-curable elastomer, (ii) an infrared dye, and (iii) microspheres, between a cover sheet and a backing sheet to form a printing plate;
b) exposing the collapsible UV-curable elastomer composition through the backing sheet to establish a floor layer;
c) removing the cover sheet from the printing plate;
d) using a laser to collapse and melt portions of the collapsible UV-curable elastomer to form a relief image on the printing plate; and
e) UV-curing said UV-curable elastomer by face exposure to crosslink said formed relief image; and
f) post-curing and detacking the plate, if required.
Additional features and advantages of the present invention will become apparent through the disclosure described below.